Fastener

ABSTRACT

A fastener has a metallic member and a nonmetallic cap member. The metallic member has a head and a shaft. The shaft has a first end at the head, a distal second end, and an externally threaded portion. The metallic member further has a projection extending from the head opposite the shaft. The cap member is mated to the head opposite the shaft and retained by the projection.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional application of Ser. No. 12/719,301, filed Mar. 8,2010, and entitled “Fastener” (now U.S. Pat. No. 8,191,224) which is adivisional application of Ser. No. 11/416,655, filed May 2, 2006, andentitled “Fastener” (now abandoned).

U.S. GOVERNMENT RIGHTS

The invention was made with U.S. Government support under contractN00019-02-C-3003 awarded by the U.S. Navy. The U.S. Government hascertain rights in the invention.

BACKGROUND OF THE INVENTION

The invention relates to aircraft. More particularly, the inventionrelates to fasteners exposed to the exhaust gas flowpath of an aircraftpowerplant.

A variety of non-metallic materials have been developed for use inaircraft propulsion. One family of materials are ceramic matrixcomposites (CMCs). In various implementations, CMCs offer advantageouscombinations of: (a) resistance to high temperatures; and (b) lowobservability. In low observability military aircraft, both theseproperties are important for use in components along the exhaust gasflowpaths of the aircraft engines. Exemplary ceramic matrix compositecomponents are nozzle flapliners and nozzle seal liners.

Such components must, typically, be secured to support structure.Typical aerospace fasteners are metallic. U.S. pre-grant publications20050260033A1 and 20050260034A1 disclose exemplary fasteners and theirapplications. If such fasteners are used to secure the CMC components,exposed metal of the fasteners may compromise low observabilityproperties.

SUMMARY OF THE INVENTION

Accordingly, one aspect of the invention involves a fastener having ametallic member and a nonmetallic cap member. The metallic member has ahead and a shaft. The shaft has a first end at the head, a distal secondend, and an externally threaded portion. The metallic member further hasa projection extending from the head opposite the shaft. The cap memberis mated to the head opposite the shaft and retained by the projection.

In various implementations, the shaft may further have a grippingfeature adjacent to the second end. The gripping feature may comprise atleast one of internal or external splines or facets. For example, thegripping feature may comprise a hexagonal-section compartment. Anunderside of the head and an underside portion of the cap may form astepped frustoconical surface. The cap may comprise or consistessentially of a ceramic matrix composite. An internally threaded nutmay be in threaded engagement with the shaft.

Such a fastener may be used to secure an engine exhaust panel to asupport structure. The panel may have a first surface exposed to anexhaust flowpath of the engine. The head and cap may be flush orsubflush in a bevel in the panel first surface. The nut may be engagedto the shaft to sandwich the panel and support structure between thehead and nut.

Another aspect of the invention involves a method for manufacturing afastener. A non-metallic cap member is mated to a projection from a headof a metallic member. The projection is unitarily formed as a singlepiece with the remaining portion of the head and a shaft of metallicmember, the shaft opposite the projection. The projection is deformed toa deformed condition retaining the cap member. The deformation maycomprise melting. The metallic member may be manufactured by a processincluding cutting an external thread on the shaft and forming acompartment in an end of the shaft opposite the projection.

Another aspect of the invention involves a fastener comprising a ceramicmatrix composite cap, a metallic member having a head and a threadedshaft, and means securing the cap to the head. The fastener may includemeans for engaging the metallic member.

Another aspect of the invention involves a method for assembling anengine exhaust panel assembly using such a fastener. The fastener shaftis inserted through an aperture in a ceramic matrix composite panel andan aperture in a support structure. The shaft is engaged to aninternally threaded nut. The nut is engaged with a tool. A feature ofthe shaft is engaged with the tool or with another tool. The nut andfastener are relatively rotated to tighten the nut to the fastener.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a fastener assembly mounting a CMC panel.

FIG. 2 is a sectional view of a stud of the assembly of FIG. 1.

FIG. 3 is a view of a precursor of a metallic portion of the stud ofFIG. 2.

FIG. 4 is a sectional view of a cap of the stud of FIG. 2.

FIG. 5 is an assembled view of the precursor and cap in an initial stageof manufacture.

FIG. 6 is a partial sectional view of a head of the stud of FIG. 2.

FIG. 7 is a sectional view of an alternate fastener assembly mounting aCMC panel.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

FIG. 1 shows a panel 20 along an exhaust flowpath 22 of a gas turbineengine 10. The panel 20 is mounted to a support structure 24 by means ofa plurality of fastener assemblies 26. The exemplary fastener assemblieseach comprise a stud 28 and a nut 30 having a common centrallongitudinal axis 500.

Each panel 20 has a first face 40 along the exhaust flowpath 22 and anopposite second face 42 engaging a first face 44 of the supportstructure 24. The engagement may be through a washer (e.g., metallic) 46or other rigid or compliant intervening component. A second face 48 ofthe support structure 24 is engaged by the nut 30 (e.g., via a secondwasher 50 and a biasing compression coil spring 54. An exemplary panelthickness between the faces 40 and 42 is 5-20 mm.

The exemplary stud 28 has a head 60 countersunk flush or near flush(e.g., slightly sub-flush by an offset 62) to the panel first face 40.The panel countersink is formed by a frustoconical bevel surface 64.

To provide desired structural security of mounting, the threaded portionof the stud may advantageously be formed of an appropriate aerospacealloy (e.g., a nickel-based superalloy). However, a large exposed headsurface of such alloy would potentially increase observability of theaircraft. For example, the exposed metal head would provide anon-negligible radar return in a tail-on view. To limit theobservability impact of the fastener, advantageously a large portion ofthe exposed surface of the head may be formed of a material selected forthis purpose.

FIG. 2 shows further details of the exemplary stud 28. The stud includesa metallic member 80 (e.g., of the nickel-based superalloy) and anon-metallic cap 82 (e.g., of CMC). The metallic member 80 comprises ashaft 86 having a distal end 88 and a proximal end 90. A shaft threadedportion 92 is located therebetween. At the proximal end 90, the shaft isunitarily formed with a head 94. The exemplary head 94 has anessentially frustoconical underside 96 and a flat outboard face 98opposite the surface 96. A slightly protuberant projection 102 extendsfrom the head opposite the shaft and has a relatively small diameterproximal portion 104 and a larger diameter distal portion 106.

The projection 102 is captured in a compartment 110 in the cap 82 toretain the cap to the head 94 so that the cap 82, head 94, andprojection 102 essentially form the stud head 60. Thus, the projectionand compartment form means of the metallic member 80 and cap 82 forsecuring the cap to the head of the metallic member. The cap has aradially inboard face 112 engaged to the head outboard face 98. Afrustoconical surface 114 extends from the face 98 and combines with thehead frustoconical underside 96 to form a slightly stepped frustoconicalstud head underside contacting the bevel surface 64. The cap has anoutboard face 116 opposite the face 112. The cap outboard face 116 formsa great majority of the surface area of the stud exposed to the exhaustflowpath 22 to provide relatively low observability and thermallyprotect the metallic member. The frustoconical surface 114 is recessedfrom the frustoconical surface 96 by an offset 118. This offsettingallows for compressive engagement between the surface 96 and the surface64 while not subjecting the surface 114 to stressing contact with thesurface 64. Accordingly, a gap 119 (FIG. 1) is of slightly smallerdimension than the recess/offset 118 and, subject to manufacturingtolerances, advantageously close to zero.

Near the distal end 88, the shaft 86 includes an engagement feature 120(means for engaging the metallic shaft 86) permitting the shaft to beengaged by a tool 122 to prevent rotation of the stud 28 duringinstallation of the nut 30 (or otherwise provide a desired relativerotation of the shaft 86 relative to the nut 30 to tighten the nut ontothe shaft). The exemplary engagement feature comprises an internalfeature such as facets 124 providing a hexagonal section distal portionof a counterbored compartment 126. An alternative engagement featuremight comprise internal splines or external splines or facets(schematically shown as 124′ in FIG. 7). Similarly, the nut 30 has anengagement feature (e.g., external hexagonal faceting 130 for wrench 132engagement).

In an exemplary method of manufacture, a precursor 140 (FIG. 3) of themetallic member 80 may be formed (e.g., by conventional screw machiningtechniques). The precursor 140 has all the portions of the metallicmember except that a precursor 142 of the projection 102 extends fromthe head outboard face 98. The precursor 142 may be formed as a shaft ofrelatively smaller diameter than the shaft 86 and greater length thanthe ultimate projection 102. As is discussed below, during subsequentassembly, a distal end portion 144 of the precursor 142 is melted toform the protuberant distal portion 106 of the projection.

The cap 82 may be made by cutting (e.g., water jet cutting) from a CMCsheet. The cap compartment 110 (FIG. 4) is formed by an outboard bevel150 extending from relatively narrow diameter inboard neck portion 152and inboard bevel 154. The inboard neck portion 152 is dimensioned toclosely accommodate the projection proximal portion 104. The exemplarycap 82 has a thickness 160 (e.g., 2-4 mm). An exemplary diameter 162 ofthe cap inboard face 112 is 10-30 mm. An exemplary half angle 164 of thefrustoconical surface 114 is 50-70°. To assemble the fastener, the cap82 is placed over the projection precursor 142 (FIG. 5) so that thedistal portion 144 of the precursor lies within or protrudes from theoutboard bevel 150. The distal portion 144 is then melted or otherwisedeformed (FIG. 6) to expand its diameter to form the projection 102 andbacklock the cap 82 to the head 94. The exemplary melting forms theprojection 102 flush or slightly sub-flush to the outboard surface 116.

One or more embodiments of the present invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention. Forexample, the fastener may be provided by reengineering the head of anyof a number of known or yet-developed fastener configurations.Accordingly, other embodiments are within the scope of the followingclaims.

1. A fastener comprising: a metallic member having the unitarily formed,single piece, combination of: a head; a shaft having: a first end at thehead; a distal second end; and an externally threaded portion; andprojection extending from the head opposite the shaft; and a nonmetalliccap member mated to the head opposite the shaft and retained by adeformation of the projection.
 2. The fastener of claim 1 wherein: theprojection is protuberant.
 3. The fastener of claim 1 wherein: theprojection has a proximal portion and a distal portion of largerdiameter than the proximal portion.
 4. The fastener of claim 1 wherein:the shaft further comprises a gripping feature adjacent the second end.5. The fastener of claim 4 wherein: the gripping feature comprises atleast one of: internal facets, internal splines, external facets, andexternal splines.
 6. The fastener of claim 4 wherein: the grippingfeature comprises a hexagonal-section compartment.
 7. The fastener ofclaim 1 wherein: an underside of the head and an underside portion ofthe cap form a stepped frustoconical surface.
 8. The fastener of claim 1wherein: the cap comprises or consists essentially of a ceramic matrixcomposite.
 9. The fastener of claim 1 further comprising: an internallythreaded nut in threaded engagement with the shaft.
 10. An engineexhaust panel assembly comprising: a panel having a first surfaceexposed to an exhaust flowpath of the engine; a support structuresupporting the panel; and a fastener according to claim 1 securing thepanel to the support structure.
 11. The assembly of claim 10 wherein:the head and cap are flush or sub-flush in a bevel of the panel.
 12. Theassembly of claim 10 wherein: a nut is engaged to the shaft.
 13. Afastener comprising: a ceramic matrix composite cap; a metallic memberhaving a head and a threaded shaft; and means for securing the cap tothe head.
 14. The apparatus of claim 13 further comprising: means forengaging the metallic member.
 15. The apparatus of claim 14 wherein: themeans for engaging the metallic member comprises an internal compartmentin a distal end of the shaft.
 16. The apparatus of claim 13 wherein: anunderside of the head and an underside portion of the cap form a steppedfrustoconical surface.
 17. The apparatus of claim 13 wherein: the meansfor securing comprises a projection of the metallic member unitarilyformed as a single piece with the head and the threaded shaft.
 18. Thefastener of claim 17 wherein: the projection is protuberant.
 19. Thefastener of claim 17 wherein: the projection has a proximal portion anda distal portion of larger diameter than the proximal portion.